Sharpening up Galactic all-sky maps with complementary data - A machine learning approach

Galactic all-sky maps at very disparate frequencies, like in the radio and $\gamma$-ray regime, show similar morphological structures. This mutual information reflects the imprint of the various physical components of the interstellar medium. We want to use multifrequency all-sky observations to test resolution improvement and restoration of unobserved areas for maps in certain frequency ranges. For this we aim to reconstruct or predict from sets of other maps all-sky maps that, in their original form, lack a high resolution compared to other available all-sky surveys or are incomplete in their spatial coverage. Additionally, we want to investigate the commonalities and differences that the ISM components exhibit over the electromagnetic spectrum. We build an $n$-dimensional representation of the joint pixel-brightness distribution of $n$ maps using a Gaussian mixture model and see how predictive it is: How well can one map be reproduced based on subsets of other maps? Tests with mock data show that reconstructing the map of a certain frequency from other frequency regimes works astonishingly well, predicting reliably small-scale details well below the spatial resolution of the initially learned map. Applied to the observed multifrequency data sets of the Milky Way this technique is able to improve the resolution of, e.g., the low-resolution Fermi LAT maps as well as to recover the sky from artifact-contaminated data like the ROSAT 0.855 keV map. The predicted maps generally show less imaging artifacts compared to the original ones. A comparison of predicted and original maps highlights surprising structures, imaging artifacts (fortunately not reproduced in the prediction), and features genuine to the respective frequency range that are not present at other frequency bands. We discuss limitations of this machine learning approach and ideas how to overcome them

Bayesian decomposition of the Galactic multi-frequency sky using probabilistic autoencoders

All-sky observations of the Milky Way show both Galactic and non-Galactic diffuse emission, for example from interstellar matter or the cosmic microwave background (CMB). The different emitters are partly superimposed in the measurements, partly they obscure each other, and sometimes they dominate within a certain spectral range. The decomposition of the underlying radiative components from spectral data is a signal reconstruction problem and often associated with detailed physical modeling and substantial computational effort. We aim to build an effective and self-instructing algorithm detecting the essential spectral information contained Galactic all-sky data covering spectral bands from $\gamma$-ray to radio waves. Utilizing principles from information theory, we develop a state-of-the-art variational autoencoder specialized on the adaption to Gaussian noise statistics. We first derive a generic generative process that leads from a low-dimensional set of emission features to the observed high-dimensional data. We formulate a posterior distribution of these features using Bayesian methods and approximate this posterior with variational inference. The algorithm efficiently encodes the information of 35 Galactic emission data sets in ten latent feature maps. These contain the essential information required to reconstruct the initial data with high fidelity and are ranked by the algorithm according to their significance for data regeneration. The three most significant feature maps encode astrophysical components: (1) The dense interstellar medium (ISM), (2) the hot and dilute regions of the ISM and (3) the CMB. The machine-assisted and data-driven dimensionality reduction of spectral data is able to uncover the physical features encoding the input data. Our algorithm is able to extract the dense and dilute Galactic regions, as well as the CMB, from the sky brightness values only.Comment: 25 pages, 8 figures, 3 tables. Submitted to Astronomy & Astrophysic

Polarized radio emission of cluster galaxies

Galaxien, die Teil eines Galaxienhaufens werden, erfahren beim Einfall in dessen Zentrum den sogenannten Staudruck. Dieser drückt das vorhandene Gas in der Galaxie in die entgegengesetzte Richtung des Einfalls und beschleunigt damit die Entwicklung der Galaxie. Über Multiwellenlängenbeobachtungen ist herausgefunden worden, dass in den verlagerten Gasschweifen effektiv Sterne gebildet werden. Um ein allgemeines Verständnis dieses Phänomens zu erlangen, welches eine Vielzahl an Galaxien im Universum betrifft, wurde zum ersten Mal die Struktur und den Einfluss der Magnetfelder in solchen Gasschweifen studiert. Mithilfe einer Beispielgalaxie konnte ein geordnetes Magnetfeld entlang des Gasschweifes gemessen werden. Dieses stimmt mit Windtunnelsimulationen gleicher Galaxien überein, indem das Magnetfeld von dem intergalaktischen Medium aufgesammelt wird. Ein solches Magnetfeld kann das rausgedrückte, kalte Gas vor Evaporation schützen und somit die Bildung von Sternen begünstigen.Galaxies that become part of a galaxy cluster experience the so-called ram pressure when they fall into its center. This pushes the existing gas in the galaxy in the opposite direction of the incidence and thus accelerates the evolution of the galaxy. It has been found via multi-wavelength observations that stars are effectively formed in the displaced gas tails. To gain a general understanding of this phenomenon, which affects a large number of galaxies in the Universe, the structure and influence of magnetic fields in such gas tails has been studied for the first time. Using an example galaxy, an ordered magnetic field was measured along the gas tail. This agrees with wind tunnel simulations of identical galaxies in that the magnetic field is collected by the intergalactic medium. Such a magnetic field can protect the pushed out cold gas from evaporation and thus favor the formation of stars

Role of Magnetic Fields in Ram Pressure Stripped Galaxies

Ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. Recently, those extended tails are found to show ongoing star formation, raising the question of how the stripped, cold gas can survive long enough to form new stars outside the stellar disk. In this study, we summarize the most recent results achieved within the GASP collaboration to provide a holistic explanation for this phenomenon. We focus on two textbook examples of jellyfish galaxies, JO206 and JW100, for which, via multi-wavelength observations from radio to X-ray and numerical simulations, we have explored the different gas phases (neutral, molecular, diffuse-ionized, and hot). Based on additional multi-phase gas studies, we now propose a scenario of stripped tail evolution including all phases that are driven by a magnetic draping sheath, where the intracluster turbulent magnetized plasma condenses onto the galaxy disk and tail and produces a magnetized interface that protects the stripped galaxy tail gas from evaporation. In such a scenario, the accreted environmental plasma can cool down and eventually join the tail gas, hence providing additional gas to form stars. The implications of our findings can shed light on the more general scenario of draping, condensation, and cooling of hot gas surrounding cold clouds that is fundamental in many astrophysical phenomena

Role of magnetic fields in ram pressure stripped galaxies

Ram-pressure stripping is a crucial evolutionary driver for cluster galaxies and jellyfish galaxies characterized by very extended tails of stripped gas, and they are the most striking examples of it in action. Recently, those extended tails are found to show ongoing star formation, raising the question of how the stripped, cold gas can survive long enough to form new stars outside the stellar disk. In this study, we summarize the most recent results achieved within the GASP collaboration to provide a holistic explanation for this phenomenon. We focus on two textbook examples of jellyfish galaxies, JO206 and JW100, for which, via multi-wavelength observations from radio to X-ray and numerical simulations, we have explored the different gas phases (neutral, molecular, diffuse-ionized, and hot). Based on additional multi-phase gas studies, we now propose a scenario of stripped tail evolution including all phases that are driven by a magnetic draping sheath, where the intracluster turbulent magnetized plasma condenses onto the galaxy disk and tail and produces a magnetized interface that protects the stripped galaxy tail gas from evaporation. In such a scenario, the accreted environmental plasma can cool down and eventually join the tail gas, hence providing additional gas to form stars. The implications of our findings can shed light on the more general scenario of draping, condensation, and cooling of hot gas surrounding cold clouds that is fundamental in many astrophysical phenomena

Multi-epoch variability of AT 2000ch (SN 2000ch) in NGC 3432

Context. AT 2000ch is a highly variable massive star and supernova imposter in NGC 3432 first detected in 2000. It is similar and often compared to SN 2009ip, and it is therefore expected to undergo a core-collapse supernova (SN) – a SN imposter of similar brightness – in the near future. Aims. We characterize the long-term variability of AT 2000ch in the radio and optical regimes with archival data reaching back to the year 1984. We use these newly reduced observations in addition to observations in the literature to restrict the mass-loss rates of AT 2000ch at multiple epochs based on different approaches, and to infer the general properties of its circumstellar nebula with respect to the detected radio brightness. Methods. We extend the known optical light curve of AT 2000ch up to the beginning of 2022 by performing point spread function (PSF) photometry on archival data from the Palomar Transient Factory and the Zwicky Transient Facility. We reduced archival radio continuum observations obtained with the Very Large Array using standard calibration and imaging methods and complemented these with pre-reduced CHANG-E

Evidence for Mixing between ICM and Stripped ISM by the Analysis of the Gas Metallicity in the Tails of Jellyfish Galaxies

none14noneAndrea Franchetto; Stephanie Tonnesen; Bianca M. Poggianti; Benedetta Vulcani; Marco Gullieuszik; Alessia Moretti; Rory Smith; Alessandro Ignesti; Cecilia Bacchini; Sean McGee; Neven Tomičić; Matilde Mingozzi; Anna Wolter; Ancla MüllerFranchetto, Andrea; Tonnesen, Stephanie; Poggianti, Bianca M.; Vulcani, Benedetta; Gullieuszik, Marco; Moretti, Alessia; Smith, Rory; Ignesti, Alessandro; Bacchini, Cecilia; Mcgee, Sean; Tomičić, Neven; Mingozzi, Matilde; Wolter, Anna; Müller, Ancl

GASP XXXIX: MeerKAT hunts Jellyfish in A2626

We present MeerKAT H I observations of six jellyfish candidate galaxies (JFCGs) in the galaxy cluster, A2626. Two of the six galaxies JW100 and JW103, which were identified as JFCGs from B-band images, are confirmed as jellyfish galaxies (JFGs). Both of the JFGs have low H I content, reside in the cluster core, and move at very high velocities (~ 3σcl). The other JFCGs, identified as non-jellyfish galaxies, are H I rich, with H I morphologies revealing warps, asymmetries, and possible tidal interactions. Both the A2626 JFGs and three other confirmed JFGs from the GASP sample show that these galaxies are H I stripped but not yet quenched. We detect H I, H α, and CO(2-1) tails of similar extent (~50 kpc) in JW100. Comparing the multiphase velocity channels, we do not detect any H I or CO(2-1) emission in the northern section of the tail where H α emission is present, possibly due to prolonged interaction between the stripped gas and the intracluster medium. We also observe an anticorrelation between H I and CO(2-1), which hints at an efficient conversion of H I to H2 in the southern part of the tail. We find that both ram-pressure stripping and H I-to-H2 conversion are significant depletion channels for atomic gas. H I-to-H2 conversion is more efficient in the disc than in the tail

GASP XXXVIII: The LOFAR-MeerKAT-VLA View on the Nonthermal Side of a Jellyfish Galaxy

Ram pressure stripping is a crucial evolutionary driver for cluster galaxies. It is thought to be able to accelerate the evolution of their star formation, trigger the activity of their central active galactic nucleus (AGN) and the interplay between galactic and environmental gas, and eventually dissipate their gas reservoirs. We explored the outcomes of ram pressure stripping by studying the nonthermal radio emission of the jellyfish galaxy JW100 in the cluster A2626 (z = 0.055), by combining LOw Frequency Array, MeerKAT, and Very Large Array observations from 0.144 to 5.5 GHz. We studied the integrated spectra of the stellar disk, the stripped tail, and the AGN; mapped the spectral index over the galaxy; and constrained the magnetic field intensity to between 11 and 18 μG in the disk and <10 μG in the tail. The stellar disk radio emission is dominated by a radiatively old plasma, likely related to an older phase of a high star formation rate. This suggests that the star formation was quickly quenched by a factor of 4 in a few 107 yr. The radio emission in the tail is consistent with the stripping scenario, where the radio plasma that originally accelerated in the disk is subsequently displaced in the tail. The morphology of the radio and X-ray emissions supports the scenario of the accretion of magnetized environmental plasma onto the galaxy. The AGN nonthermal spectrum indicates that relativistic electron acceleration may have occurred simultaneously with a central ionized gas outflow, thus suggesting a physical connection between the two processes